Semiconductor single crystals are commonly fabricated using the Czochralski method. The semiconductor charge, e.g. silicon, is melted in a crucible made, e.g., of silica, by using heating element(s) around the crucible in a chamber. An inert gas flow, typically argon, is used to flush the furnace and the melt surface. A pulling mechanism is located above the crucible to pull the crystal from the melt. The heat-resistant parts inside the main vacuum chamber of the crystal pulling furnace, which forms the so-called hot-zone, are typically made of graphite and thermally insulating soft and/or rigid graphite felt. Various other materials (e.g. metals, composites or refractory materials) or coatings (e.g. SiC on graphite) are known to be used to some extent in the hot-zone. The crucible, heater and insulated tubular heat shield around the heater are some of the typical parts of the hot-zone in known techniques. It is also known in a basic crystal growing process that magnetic field may be used to control melt flow and/or crystal properties, e.g. oxygen concentration, and to improve the growth yield. Use of an additional bottom heater located under the crucible to be used during melting and/or growing of the crystal, to shorten the melting time and to optimize the temperature distributions in the hot-zone, is also known before as such.
The hot-zone design has an important effect on the total cost and quality of the crystals and productivity. However, the earlier hot-zones typically had a design that led to very high heat loss and heating power consumption because of the limited or locally missing thermal insulation, and at the same time, the design also led to poor gas flow characteristics that increased the gas consumption while still giving a relatively low crystal yield because of particle formation in harmful positions and reaching the melt, low lifetime of the hot-zone parts because of strong reactions at their surfaces and/or low quality of the crystals with respect to contamination or other quality aspects. The resulting relatively high cost for electricity, gas and graphite parts, combined with poor crystal yield and quality as well as the extra time needed for cleaning the furnace or for replacing the hot-zone parts, increased the total production cost per usable crystal length. Furthermore, the lowered productivity has been problematic. Additional problems can be related to the lack of stability/process reproducibility because of the corrosion/erosion of graphite parts of the current hot-zones, and/or changes in properties of various surfaces, because of deposition of e.g. silicon containing vapors. Although several improvements in the hot-zone designs, to improve some of these aspects, have been made since and will be discussed below, the situation is still far from optimized.
In the prior hot-zone designs, not much attention has been paid to the power consumption while other aspects such as quality, productivity and yield have been more in the focus. The designs have often had very limited thermal insulation. Large areas in the upper parts of the hot-zone and often also in the lower parts may have only modest insulation, and thermal leakage through, e.g., the heater electrodes and crucible shaft lead-through and gas outlet from the hot-zone to the pump-line are very significant, or the thermal insulation has not even been specified at all in some areas/locations.
The improvements of the thermal insulation have often been local, concentrating in the region above the melt, and driven by the target to improve the crystal quality and/or to increase the pulling rate while the power consumption has not been much of concern and has not been properly addressed. However, although these designs partially shield the crystal from the thermal radiation from the hot melt and the hot-zone to allow higher pulling rates, they are not optimized or targeted to decrease the power consumption of the hot-zone as a whole. High power loss and power consumption also typically lead to higher maximum temperatures, at least locally, and to larger temperature non-homogeneity inside the hot-zone. These are harmful because they lower the lifetime of the hot-zone parts or cause a drift of the temperature distribution in the hot-zone and the crystal because of increased or locally enhanced reactions. As these changes become too large, the parts have to be replaced, which increases the cost and requires extra work and time, decreasing the productivity. Higher temperatures at the crucible-melt interface are also harmful to the crucible as they speed up the unfavorable changes in the crucible and also make the melt flow behavior less stable and lead to lower crystal yield.
The inert gas is, in most cases, introduced to the hot-zone from above, passing by the crystal and the melt surface, and finally, after passing through the hot-zone, flowing through outlets to the vacuum pump lines connected to the lower parts of the main vacuum chamber of the crystal grower. Earlier hot-zones did not much pay attention the gas flow geometry in the regions close to the melt surface while in modern known hot-zones a tubular or conical part above the melt is often used for this purpose, see, e.g., U.S. Pat. No. 4,330,362 as a solution to the hot-zone related problems. However, although guiding of the gas flow through such a part offers several advantages, it may also intensify the evaporation of species from the melt to a level where this becomes a problem. Furthermore, as most of the gas flow passes the melt surface, the accidental particles, which are released from the inner surfaces of the vacuum chamber above the hot-zone or from the upper surfaces of the hot-zone, have a large probability to be transported with the gas flow to reach the melt region where these particles can lead to dislocations in the crystal and, thus, lower yield, if they reach the melt and the melt-crystal interface. Such particles on the surfaces often originate from the materials used for the hot-zone, from reactions or condensation inside the furnace, or from the cleaning or maintenance operations. In addition to particulate-type contamination, vaporized contamination from various surfaces is detrimental, too, if it gains access to the melt surface.
The lifetime of graphite parts and methods to increase the lifetime by suitable gas flow routes and hot-zone design has been discussed in the known technology. However, the known gas routing solutions represent stand-alone solutions, in which the gas flow has been separated from the heater and/or the crucible, to lengthen their lifetime and/or the lifetime of other graphite-based parts, with no concern to e.g. temperature distribution. Furnaces of known designs, however, typically need a non-standard position of the exhaust line connection to the chamber, also special equipment for the gas system, and are not easily adopted for standard furnaces without major modifications of the furnace and the crystal growing process. Such designs are not easily adaptable as such to standard types of furnaces, and may have adverse impact on the potential auxiliary systems to be used in various processes.
After growing a crystal the furnace and the hot-zone have to be opened for cleaning or maintenance operations, which include for instance the removal of the used e.g. quartz or silica crucible and the residual material therein and possible removal of dust and other debris. The condition of the hot-zone can be checked and the furnace finally charged for the next batch. The ease of opening and handling of the hot-zone is a factor contributing to the productivity and to the total cost of the crystal growing, especially in the case of larger hot-zones, as there are large and heavy parts that cannot easily be lifted by hand, but this problem has not been much addressed in the literature so far. External apparatus could be used for lifting, but it is an expensive solution and requires some space and time for docking and undocking of the apparatus to the grower and/or to the hot-zone parts. Especially the handling of the hot-zone parts above the crucible in the known furnaces slower the cleaning and other required operations, and thus the production.
Flexibility of production often requires that the same grower is used to produce different crystal diameters. For example, if the same furnace is used to grow 8″ and 6″ silicon or germanium crystals, one hot-zone design is probably not useful nor optimized for both crystal diameters. The growth of a 6″ crystal from a hot-zone design for 8″ without any changes would lead to higher cost, lower productivity, lower crystal quality and/or lower crystal yield. There is a need for a hot-zone design and procedures in which only a minimum number of small, relatively inexpensive hot-zone parts are easily and quickly changed in the adaptation from one diameter to another.